1. Field of the Invention
The present invention relates to a display device, and in particular, to a display device employing hybrid structure formed of both a TFT using poly-Si and a TFT using an oxide semiconductor.
2. Description of the Related Art
In a liquid crystal display device, a TFT substrate on which pixels each having pixel electrodes, a thin-film transistor (TFT), etc. are formed like a matrix and a counter substrate are arranged to face each other and a liquid crystal is sandwiched between the TFT substrate and the counter substrate. The liquid crystal display device forms an image by controlling the light transmittance of liquid crystal molecules in regard to each pixel. On the other hand, an organic electroluminescence (EL) display device forms a color image by use of a self-luminous organic EL layer and a TFT that are arranged in each pixel. The organic EL display device needs no backlight, and thus is advantageous for the thinning of the device.
Low temperature poly-Si (LTPS) has high carrier mobility and thus is suitable as a TFT for a drive circuit. In contrast, oxide semiconductors have high OFF resistance, and the OFF current of a TFT can be reduced by using an oxide semiconductor for the TFT.
JP-A-2013-175718 and JP-A-2011-54812 can be taken as examples of prior art literature having a description of a TFT using an oxide semiconductor. JP-A-2013-175718 describes a configuration in which metallic oxide is formed on an oxide semiconductor constituting the channel and is used as a gate insulation film. JP-A-2011-54812 describes the use of a metallic oxide layer or a semiconductor layer as a sacrificial layer for channel etching in a bottom-type TFT using an oxide semiconductor